I. Field of the Invention
The present invention relates to digital wireless communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for performing accurate open loop power control in a code-division multiple access (CDMA) communication system.
II. Description of the Related Art
In the field of wireless communications, several technology-based standards exist for controlling communications between a mobile station, such as a cellular telephone, Personal Communication System (PCS) handset, or other remote subscriber communication device, and a wireless base station. These include both digital-based and analog-based standards. For example, among the digital-based cellular standards are the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) Interim Standard IS-95 series including IS-95A and IS-95B, entitled "Mobile Station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System." Similarly, among the digital-based PCS standards are the American National Standards Institute (ANSI) J-STD-008 series, entitled "Personal Station--Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems."
The spread spectrum modulation technique of CDMA has significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention and incorporated by reference herein.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile station through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31, 1992, entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the present invention and incorporated by reference herein.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile station power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", also assigned to the assignee of the present invention and incorporated herein by reference. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein.
Fundamental to the concept of cellular or PCS telephony in a CDMA system is the process of power control. A CDMA communication system is an interference limited system in the sense that the system capacity (i.e., the number of simultaneous calls) is a function of the maximum amount of interference that the system can tolerate. The amount of interference depends on the received signal level from each mobile station. The output power of each mobile station must be controlled so as to guarantee enough signal strength received at the base station to maintain good signal quality. The system capacity is maximized if the transmit power of each user is controlled so that the signal power at the cell receiver is the minimum required to achieve a predetermined signal-to-noise ratio (SNR). Hence, to achieve high capacity, the CDMA system employs power control on both forward and reverse links to solve the near-far problem, the corner problem, and long and short term channel variations.
In the open loop power control method according to IS-95, the mobile station uses the measured total received power along with typical values of certain base station parameters to get a rough estimate of the transmission loss between the unit and the base station. Based on these measurements, the forward link transmission loss is estimated and used to determine the proper open loop power control setting for the mobile station transmitter. The mobile station's transmit power is adjusted to match the estimated path loss, to arrive at the base station at a predetermined level. All mobile stations use the same process, and ideally their signal will arrive with equal power at the base station.
The base station parameters just mentioned are passed to the mobile station over the paging channel from the base station to the mobile station in an Access Parameters Message, described in detail in section 7.7.2.3.2.2 of IS-95. Of particular importance to the present invention are the fields NOM.sub.-- PWR and INIT.sub.-- PWR which are contained in the Access Parameters Message. The field NOM.sub.-- PWR represents a correction factor to be used by the mobile station in its open loop power estimate, and the field INIT.sub.-- PWR represents a correction factor to be used by the mobile station in the open loop power estimate for the initial transmission on an access channel.
The current algorithm for estimating the mobile station's desired transmit power for the first access probe on the access channel is given by: ##EQU1## For subsequent probes on the access channel, each access probe sequence is sent at an increased power level until a response is obtained or the sequence ends. The amount of each increase is set by the field PWR.sub.-- STEP, which is also sent in the Access Parameters Message. Then, the initial transmit power on the reverse traffic channel is specified to be: ##EQU2##
The values for NOM.sub.-- PWR, INIT.sub.-- PWR, and PWR.sub.-- STEP are all defined by the base station, and sent to the mobile station prior to the mobile station's first transmission. The nominal value of NOM.sub.-- PWR is 0 dB, with a range of -8 to 7 dB. The nominal value of INIT.sub.-- PWR is also 0 dB with a range of -16 to 15 dB. The range of PWR.sub.-- STEP is 0 to 7 dB.
Looking at Equations (1) and (2), one can see that a constant of -73 dB is used as the open loop power control constant. This value is a nominal value which was calculated as follows: ##EQU3## evaluated at the fixed nominal values of: ##EQU4## Bit energy to total noise power spectral density=7 dB k=Boltzmann's constant=1.38.times.10.sup.-23 Joule/Kelvin
T=Temperature of the receiver in Kelvin=290.degree. K. PA0 R=Data Rate=9.6 Kbps PA0 F=Base station noise figure=5 dB PA0 P.sub.t.sup.c =Total base station transmit power=20 Watts PA0 X=Reverse link loading factor=50% PA0 M=Equivalent number of cells surrounding the mobile station=2 PA0 .zeta..sub.i =Ratio of power received from cell i to power received from mobile station's home cell=1.
From the above calculations, it is clear that the value of the open loop power control constant depends on many dynamically varying parameters. For example, the total base station transmit power may vary depending on whether the cell's effective radiation power (ERP) has been recently adjusted. The ratio of power received from cell i to the power received from the mobile station's home cell may vary depending on the location of the mobile station within the cell. When the mobile station is at the edge of its home cell's coverage, the ratio may increase. Conversely, when it is closer to the center of the home cell, the ratio may decrease. The number of cells surrounding the mobile station is clearly not the same for every cell. Some cells are more remotely located than others and will have fewer surrounding cells. The cell receiver noise figure may change due to maintenance or upgrading of the receiver. The reverse link Eb/Nt may change simply due to the nature of the dynamic wireless environment.
None of these dynamically changing variables is known, a priori, to the mobile station. Hence, the first probe power level will likely be in error because of the use of nominal values rather than actual values in calculating the open loop power control constant. As a result, when the mobile station is close to the base station, it will transmit at far too high of a power level than necessary to establish communications. To the extent that the transmit power level is too high, it constitutes unnecessary interference to the remaining mobile stations, reducing the capacity of the system. On the other hand, if the mobile station is far away, it may transmit the initial access probe at too low a power level, resulting in additional probes being sent. In addition to increasing call setup time, additional probes will result in more reverse link interference. What is needed is an improved method and apparatus for performing accurate open loop power control in a code-division multiple access (CDMA) communication system, which will increase the probability of successful completion of the first access probe and hence decrease the amount of reverse link interference.